The supply of carbon dioxide from natural sources, by-product CO.sub.2 from ammonia manufacture, and hydrogen purification, is not sufficient for the present and future industrial requirements.
The potential supply of CO.sub.2 from power plant flue gas could furnish the required amount providing it could be economically recovered. Flue gas normally will be at or near atmospheric pressure and contains about 6-10% CO.sub.2 and about 2-5% oxygen. Sulfur dioxide may be an additional contaminant if the fuel source is coal instead of sweet or commercial natural gas.
Most known solvents that can recover CO.sub.2 under these conditions will undergo severe solution oxidative degradation and cause corrosion, thus rendering the process uneconomical.
The removal of carbon dioxide from flue gas was practiced in the `50`s and early `60`s by extracting the carbon dioxide from the combustion products resulting from burning a fuel. In addition inert atmospheres for large annealing furnaces were produced in a like manner.
The principal solvent used in the removal of CO.sub.2 from flue gas during this same period employed an aqueous monoethanolamine (MEA) solution in the concentration range of 5-12%. The system was operated until oxidative degradation products and corrosion became sufficiently severe as to warrant discarding the solution with plant cleanup and recharging with fresh solution.
Some processes were improved, vis-a-vis using only the dilute MEA solution until it went bad by operating a side stream reclamation still. Such still removed some of the oxidative degradation products as a bottom product while taking substantially the MEA and water as an overhead product for recycle. The side stream still operated on a 2-3% side stream. This approach was not particularly successful because the degradation products were removed only to a limited extent in the side stream reclaimer. In addition, degradation products continued to be produced at a higher rate due to the high temperatures necessary for operating the reclaimer still.
Another mode of operation of the dilute solution process was the utilization of a 5-8% aqueous MEA solution with a 4-8% concentration of sodium carbonate. Sodium carbonate neutralized degradation products that were acidic in nature (formic acid is the prime oxidation product in this environment). This mode of operation was somewhat successful but like the other two mentioned systems, was unpredictable in the length of time the system would operate before losing capacity to recover CO.sub.2.
All the processes mentioned above were extremely energy intensive due to the extremely high circulation rates necessitated by the low concentration of MEA and the very low loadings of CO.sub.2 that were permitted in order to minimize corrosion.
The recovery of CO.sub.2 from a flue gas using a combustion zone to lower residual oxygen is described in U.S. Pat. No. 4,364,915 dated Dec. 21, 1982.
A mode of operation utilizing copper salts as an inhibitor is disclosed in U.S. Pat. No. 2,377,966, dated June 12, 1945. This method was used in the above mentioned systems that did not include the use of a reclaimer in the operation. Copper was only moderately successful as a corrosion inhibitor even at the low CO.sub.2 loadings and low concentrations of alkanolamine. Precipitation of elemental copper was a serious limitation of this process and resulted in enhanced corrosion due to galvanic attack in the peripheral area of the deposited copper metal. This system was operated much the same way that the uninhibited aqueous MEA solution first mentioned was utilized, in that when the system became sufficiently degraded the entire solution was dumped, the internals of the plant cleaned, fresh alkanolamine charged back to the system, and the system put back in service. The length of time the system remained on stream was again unpredictable.
The use of activated carbon or ion exchange resin to remove contaminates from aqueous alkanolamine solutions is known from U.S. Pat. Nos. 1,944,122; 2,797,188; 3,568,405; and 4,287,161. However, these patents do not suggest the suprising results obtained herein using an effective amount of copper salts in the alkanolamine solution in conjunction with the use of activated carbon or ion exchange resin.